Ron is correct. I would add that the former are often characterized as "crew cars" but serve the same purpose as courtesy cars. The nicer FBOs make them available for the comparatively poor crew members who make the expensive fuel purchasing decisions for their deeper pocketed clients or employers. The FBOs seem to begrudgingly make them available to us spam canners when politely asked, but often require completing essentially a zero dollar rental agreement. These types of cars are more likely to not be available to younger pilots. If I was flying into a Signature-type FBO, I'd call ahead to check on age restrictions for the car (as well as fuel prices, handling fees, etc.) Alternately, the old minivan retired cop car etc. courtesy cars, like others have mentioned, are pretty easy to get for the asking. Just don't act like a tool and most won't even notice that you're 18.